Cylinder apparatus and method of manufacturing rod for cylinder apparatus

ABSTRACT

A groove processing method capable of smoothly processing a groove on a rod that has been subjected to a hardening treatment without a substantial degradation in strength is provided. A pair of straightening rollers is mounted at each side of rolling rollers mounted on respective rotating shafts of a rolling machine with spacers disposed between the straightening and rolling rollers. A rod is previously subjected to a hardening treatment, for example, high-frequency induction hardening and tempering, or carbulizing &amp; quenching and tempering, so that the surface hardness is not less than 400 Hv. A groove is formed on the rod by rolling using the rolling rollers, thereby allowing a surface hardening layer to be left on the bottom of the groove. At the same time, the rod is held by the straightening rollers to prevent bending of the rod.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a cylinder apparatus such as a hydraulic shock absorber, a hydraulic cylinder for an active suspension system, and a gas spring, and also relates to a method of manufacturing a rod for use in such a cylinder apparatus.

[0002] For example, a hydraulic shock absorber has a structure as shown in FIG. 7. An inner cylinder 2′ having a piston 1′ slidably fitted therein is accommodated in an outer cylinder 3′, one end of which is closed. A rod (piston rod) 4′ is connected at one end thereof to the piston 1′. The other end portion of the rod 4′ extends to the outside through a rod guide 5′ fitted mutually to the respective opening end portions of the inner cylinder 2′ and the outer cylinder 3′. A hydraulic fluid sealed in the inner cylinder 2′ is caused to flow through piston valves 6′ provided on the piston 1′ and also through a base valve 7′ provided in the bottom of the outer cylinder 3′, whereby damping force is generated during the extension stroke and the compression stroke. A reservoir R′ formed between the inner cylinder 2′ and the outer cylinder 3′ has a gas and hydraulic fluid sealed therein to compensate for a change in the amount of hydraulic fluid in the inner cylinder 2′ corresponding to the amount by which the piston rod 4′ enters or withdraws from the inner cylinder 2′. It should be noted that reference numeral 8′ denotes a mounting eye used to mount the hydraulic shock absorber to an axle-side member of a vehicle, and reference numeral 9′ denotes a spring retainer for retaining a coil spring.

[0003] Incidentally, a stopper member 10′ is secured to an intermediate portion of the extending end of the rod 4′. When the rod 4′ extends, the stopper member 10′ abuts against the rod guide 5′ to control the extension of the rod 4′. A resin member (not shown) having elasticity, e.g. rubber, is secured to the top of the stopper member 10′. The resin member damps shock applied when the stopper member 10′ abuts against the rod guide 5′. In this related art, projection welding is employed as a method of securing the stopper member 10′ to the rod 4′.

SUMMARY OF THE INVENTION

[0004] However, the above-described method wherein the stopper member 10′ is joined to the rod 4′ by projection welding needs to increase the welding current in order to obtain a desired joint strength. As the welding current increases, the amount of spatter generated during welding increases. The increased amount of spatter is likely to adhere to various portions of the rod 4′. If spatter adhering to various portions of the rod 4′ enters the hydraulic fluid chamber as contamination during assembling of the hydraulic shock absorber, which is carried out afterward, the contamination may get caught in the valves, thus making it impossible to obtain desired damping force characteristics. To prevent this problem, a contamination removal operation needs to be performed. This causes the number of working man-hours to increase unfavorably.

[0005] Under these circumstances, the following technique may be conceived. A groove is formed on the surface of an intermediate portion of the rod 4′, and a collar is fitted in the groove by using a metal flow joining process, for example. The collar is used as a substitute for the stopper member 10′, thereby omitting the welding process.

[0006] However, the rod 4′ has usually been subjected to a hardening treatment consisting of high-frequency induction hardening and tempering. As a result, the surface of the rod 4′ has a high hardness, i.e. not less than 400 in terms of Vickers hardness (Hv). Therefore, cutting (turning) unavoidably has to be used to form the groove with general techniques falling under the category of prior art. In this case, the surface hardening layer formed by the hardening treatment is undesirably removed. This causes a reduction of area and a substantial degradation in strength, particularly in bending strength.

[0007] Further, because the rod 4′ is difficult to cut, the lifetime of the tool for cutting is unfavorably short.

[0008] An object of the present invention is to provide a cylinder apparatus and a method of manufacturing a rod for use in a cylinder apparatus, which have been invented in view of the above-described technical background.

[0009] The present invention provides a cylinder apparatus including a cylinder having a fluid sealed therein. A rod is inserted in the cylinder. The rod is formed from a circular cylinder-shaped base metal having a circumferential groove formed by rolling after the base metal has been subjected to a hardening treatment. An engagement member is secured to the rod by engaging in the circumferential groove on the rod. In the cylinder apparatus, a surface hardening layer can surely be left on the bottom of the groove as well. Accordingly, sufficient strength can be ensured for the rod.

[0010] The surface hardness of the rod is preferably not less than 400 in terms of Vickers hardness (Hv). Such a surface hardness minimizes swelling of material at both sides of the groove during rolling.

[0011] The rolling process should preferably be carried out in such a manner that while the circumferential groove is being formed by using rolling rollers, possible axial bending of the rod is corrected with straightening rollers having a flat peripheral surface, which are disposed at both sides of the rolling rollers in the axial direction thereof. With this process, bending of the rod is suppressed by the straightening effect of the straightening rollers.

[0012] In addition, the present invention provides a method of manufacturing a rod for use in a cylinder apparatus wherein the rod is inserted into a cylinder having a fluid sealed therein. The cylinder apparatus has an engagement member that is engaged in a circumferential groove formed on the rod. According to the rod manufacturing method, a circular cylinder-shaped base metal is subjected to a hardening treatment so as to be hardened. Then, the circumferential groove is formed on the base metal by rolling. Thereafter, the base metal is subjected to a surface hardening treatment. This method allows a surface hardening layer to be surely left on the bottom of the groove as well and makes it possible to produce a rod ensured sufficient strength.

[0013] It is preferable that the surface hardness of the rod obtained by the hardening treatment should be not less than 400 in terms of Vickers hardness (Hv). Such a surface hardness minimizes swelling of material at both sides of the groove during rolling.

[0014] The rolling process should preferably be carried out in such a manner that while the circumferential groove is being formed by using rolling rollers, possible axial bending of the rod is corrected with straightening rollers having a flat peripheral surface, which are disposed at both sides of the rolling rollers in the axial direction thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a sectional plan view showing the structure of a rolling machine used in a roller rolling process for forming a groove on a rod according to the present invention, and also showing the way in which rolling is carried out.

[0016]FIG. 2 is a front view schematically showing the way in which rolling is carried out by the rolling machine shown in FIG. 1.

[0017]FIG. 3 is a sectional view showing a groove configuration formed by the roller rolling process according to the present invention and the condition of a surface hardening layer.

[0018]FIG. 4 is a side view showing a mode of using a rod that is to be processed by the present invention.

[0019]FIG. 5 is a graph showing the hardness distribution of a surface portion of the rod after the groove forming process according to the present invention has been carried out in comparison to the hardness distribution of a general portion of the rod.

[0020]FIG. 6 is a sectional view showing the general structure of a hydraulic shock absorber equipped with a rod that is to be processed by the present invention.

[0021]FIG. 7 is a sectional view showing the general structure of a hydraulic shock absorber according to the related art.

DETAILED DESCRIPTION OF THE INVENTION

[0022] An embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

[0023] A cylinder apparatus according to this embodiment will be described by using FIGS. 1 to 6. It should be noted that, as shown in FIG. 6, the hydraulic shock absorber according to this embodiment is not different in external appearance from the above-described hydraulic shock absorber according to the related art shown in FIG. 7.

[0024] In this embodiment, a rod (piston rod) 4 shown in FIG. 6 is provided with a circumferential groove 15 as shown in FIGS. 3 and 4 by rolling. The groove 15 is eventually fitted with a collar 16 that is secured to the rod 4 as an engagement member in place of the stopper member 10′ (see FIG. 7). The collar 16 is fitted to the groove 15, for example, by using a metal flow joining process, as shown in FIG. 4, whereby the collar 16 is transformed from the state of being the collar 16 a to the state of being the collar 16 b. Thus, the collar 16 is engaged in the groove 15.

[0025] In the above-described related art, a resin member having elasticity, e.g. rubber, is secured to the top of the stopper member 10′. In this embodiment, the lower end of a rebound spring (not shown in FIG. 6) is engaged with the top of the collar 16.

[0026] The rod 4 is previously subjected to a hardening treatment, for example, high-frequency induction hardening and tempering, or carburizing & quenching and tempering, to harden a surface layer closer to the surface of the base metal so as to increase the strength of the base metal. As shown in FIG. 3, the hardened surface portion has a surface hardening layer 17 with a Vickers hardness of not less than 400 Hv, which is formed over a predetermined width (on the order of 0.7 to 1.0 mm, by way of example).

[0027] In this embodiment, roller rolling is employed as a method of forming the groove 15 on the rod 4. The roller rolling is performed by using a rolling machine, as shown in FIGS. 1 and 2. The rolling machine has two rotating shafts 21A and 21B arranged in parallel to each other and rolling rollers 22A and 22B mounted on the rotating shafts 21A and 21B, respectively. The rod 4 as a work supported by work support blades 23 is disposed between the rolling rollers 22A and 22B. While the rolling rollers 22A and 22B are being rotated in the same direction, one rolling roller (movable roller) 22A is fed toward the other rolling roller (fixed roller) 22B, as indicated the arrow F, to roll the rod 4 between the rolling rollers 22A and 22B. The peripheral surfaces of the rolling rollers 22A and 22B are provided with annular convex seats 24A and 24B, respectively. The peripheral surfaces of the convex seats 24A and 24B are provided with annular forming ridges 25A and 25B, respectively. When the rod 4 is rolled between the rolling rollers 22A and 22B, the forming ridges 25A and 25B are caused to penetrate or be forced into the peripheral surface of the rod 4, whereby the groove 15 is formed on the surface of the rod 4.

[0028] In this case, the rod 4 has a higher distortion resistance at the surface side than at the center thereof because of the presence of the surface hardening layer 17. Therefore, the surface hardening layer 17 is pushed into the bottom of the groove 15 while substantially keeping its thickness as it is, as shown in FIG. 3. In other words, the surface hardening layer 17 is also present on the bottom of the groove 15 with a sufficient thickness. Accordingly, the strength increasing effect produced by the surface hardening treatment can be maintained as it is at the groove 15 of the rod 4 as well. As a result, strength necessary for the rod 4 is ensured satisfactorily. It should be noted that the groove 15 has a parallel surface 15 a provided at the deepest portion. A curved surface 15 b is provided at one side of the parallel surface 15 a. A tapered surface 15 c is provided at the other side of the parallel surface 15 a. Thus, the groove 15 has an irregular sectional configuration in comparison to a rectangular sectional configuration. The groove 15 is formed in such an irregular sectional configuration in view of joinability required when the collar 16 is joined to the groove 15 by a metal flow process, and also considering the direction of an input load to the collar 16.

[0029] Incidentally, if the work used in the above-described roller rolling process is a softened material (annealed material), which has not been subjected to a hardening treatment, an excess of metal corresponding to the amount of penetration of the forming ridges 25A and 25B of the rolling rollers 22A and 22B forms swells at both sides of the convex seats 24A and 24B. Thus, the sectional stress distribution of the work becomes uniform. Therefore, there is no possibility of the work being bent after rolling, not to mention during rolling. In this regard, we actually performed a manufacturing experiment. As a result, swells were formed at both sides of the convex seats 24A and 24B when the Vickers hardness Hv was in the range of from that of the crude metal to about 350. Thus, a rod suitable for use as a product could not be obtained.

[0030] However, with a work that has been subjected to a hardening treatment so as to have a Vickers hardness of not less than 400 Hv as in the case of the rod 4, to which the invention of this application is directed, swelling of material at both sides of the convex seats 24A and 24B is minimized by the increase in deformation resistance at the surface side as stated above. Thus, the rod processed in this way is at a level suitable for use as a product. However, an excess of metal flows in the axial direction, and the whole rod 4 extends in the axial direction, causing the sectional stress distribution to become irregular. Accordingly, the whole rod 4 is likely to bend at the groove 15 toward a side where the extension is smaller.

[0031] Therefore, in this embodiment, a pair of straightening rollers 27A and 27B are mounted on the rotating shafts 21A and 21B, respectively, at each side of the corresponding rolling rollers 22A and 22B with spacers 26A and 26B interposed therebetween, and the straightening rollers 27A and 27B are rotated together with the rolling rollers 22A and 22B as one unit. Each pair of straightening rollers 27A and 27B have a flat peripheral surface and an outer diameter set equal to or slightly smaller than the outer diameter (reference outer diameter) of the convex seats 24A and 24B of the rolling rollers 22A and 22B. Each pair of straightening rollers 27A and 27B have respective peripheral edge portions 28A and 28B chamfered into a predetermined radius (R) shape.

[0032] The straightening rollers 27A and 27B disposed at both sides of the rolling rollers 22A and 22B hold portions of the rod 4 that are located at both sides of the portion to be rolled during rolling by the rollers 22A and 22B. Therefore, bending due to the above-described material flow in the axial direction is suppressed. Thus, a rod 4 excellent in straightness is obtained. Further, because the peripheral edge portions 28A and 28B of the straightening rollers 27A and 27B have been chamfered, the rod 4 will not be damaged or stepped.

[0033] It should be noted that there is another method of obtaining straightness wherein bending of the rod is corrected, for example, by post-processing using a straightening machine known as “2-roll rotary straightener” having an oblique intersection angle set at 10 or 20 degrees. With this method, however, it becomes difficult to hold the configuration of the groove 15 formed by the convex seats 24A and 24B of the rolling rollers 22A and 22B (i.e. the configurations of the parallel surface 15 a, the curved surface 15 b and the tapered surface 15 c) within specified dimensions. In addition, the groove 15 needs to be reprocessed. Thus, this method is inefficient. Accordingly, the method of processing the groove 15 while holding the rod 4 with the straightening rollers 17A and 27B as in the foregoing embodiment is extremely useful for obtaining the groove configuration with high efficiency and high accuracy.

[0034] Here, a steel bar with a diameter of 20 mm made of JIS S45C (G4051) was used as a stock for a rod, and the rod was subjected to a standard hardening treatment consisting of high-frequency induction hardening and tempering. The rod thus subjected to the high-frequency induction hardening and tempering treatment was subjected to roller rolling by the above-described rolling machine having the rolling rollers 22A and 22B and the straightening rollers 27A and 27B. Thus, a groove 15 having a maximum depth (depth of the parallel surface 15 a) of about 0.7 mm and a maximum width of about 4 mm was rolled on the rod. After the rolling process, the hardness of the rod was measured along a section line P located in the deepest portion of the groove 15 on the rod 4 and along a section line Q located in the general portion of the rod 4 by using a microvickers hardness tester under a load of 200 g to determine hardness distributions. Thus, results as shown in FIG. 5 were obtained.

[0035] The results shown in FIG. 5 reveal that the depth of the surface hardening layer at the portion (P) of the groove 15 is slightly shallower than that at the general portion (Q), but a region where the hardness is not less than 400 Hv reaches a depth in the neighborhood of 0.7 mm from the surface in the portion (P). Thus, there is no problem at all in ensuring the required strength.

[0036] The rod 4 that is formed with the circumferential groove 15 in this way is further subjected to a surface hardening treatment by hard chrome plating or gas softnitriding treatment in order to prevent the rod surface from rusting or being damaged. Then, the rod 4 is subjected to a treatment for smoothing the surface by buffing or the like.

[0037] Thereafter, as shown in FIG. 4, the collar 16 is fitted to the groove 15 by using a metal flow joining process so that the collar 16 is transformed from the state of being the collar 16 a to the state of being the collar 16 b. Thus, the collar 16 is engaged in the groove 15 to complete the desired rod.

[0038] In the foregoing embodiment, the rod is manufactured in the following sequence. First, the hardening treatment is performed. Then, the groove 15 is formed. Finally, the surface hardening treatment is carried out. The reason for this is as follows. If the groove 17 is formed before the hardening treatment is performed by high-frequency induction hardening and tempering, the groove 17 cannot sufficiently be subjected to the high-frequency induction hardening and tempering treatment. Consequently, the desired strength cannot be obtained (i.e. it is desirable for the high-frequency induction hardening and tempering treatment that the surface should be flat). For this reason, the groove 17 is formed after the hardening treatment.

[0039] The hard chrome plating or the gas softnitriding treatment, which may be performed as a surface hardening treatment, forms a hardening layer of the order of 0.01 mm to 0.02 mm on or in the surface layer (hard chrome plating deposits a hardening layer on the surface layer of the base metal; gas softnitriding allows the surface layer of the base metal itself to become a hardening layer). If the formation of the groove 17 is performed after the surface hardening treatment, because the hardening layer is thin, it may be cracked. Therefore, it is desirable that the surface hardening treatment should be performed after the formation of the groove 17. The hard chrome plating and the gas softnitriding treatment can be satisfactorily carried out even if there are some irregularities on the surface. The surface hardening treatment is different in purpose from the hardening treatment for increasing the strength of the base metal. Therefore, even if the surface hardening treatment fails to be performed as desired, the engagement member can fit into the groove 17 without a particular problem. Thus, it is most desirable that the rod should be processed in the following sequence: the hardening treatment; the formation of the groove; and the surface hardening treatment.

[0040] Although the foregoing embodiment has been described by using the hydraulic shock absorber shown in FIG. 6 as a cylinder apparatus, it should be noted that the present invention is not necessarily limited to the hydraulic shock absorber but may be applied to a hydraulic cylinder for an active suspension system and also applied to a gas spring wherein a gas is sealed in a cylinder.

[0041] Although the collar 16 to which a rebound spring is fitted has been shown as an engagement member, the engagement member in the present invention is not necessarily limited thereto but may be any member that engages in a circumferential groove on a rod such as a piston. The engagement member may also be a member mounted on the outside of the cylinder, e.g. a dust cover.

[0042] Although the foregoing embodiment has been described with regard to an example in which a solid rod is used, it should be noted that the present invention is not necessarily limited thereto. It is also possible to use a hollow rod.

[0043] As has been detailed above, the cylinder apparatus according to the foregoing embodiment uses a rod having a groove formed by rolling after the rod has been subjected to a hardening treatment. Therefore, a surface hardening layer can surely be left on the bottom of the groove as well. Accordingly, sufficient strength can be ensured for the rod.

[0044] In addition, the cylinder apparatus uses a rod subjected to a hardening treatment, for example, high-frequency induction hardening and tempering, or carbulizing & quenching and tempering, so that the surface hardness of the rod is not less than 400 Hv. Therefore, it is possible to minimize swelling of material at both sides of the groove during rolling and hence possible to provide a rod even more suitable as a product to be manufactured.

[0045] Further, the rolling process is carried out in such a manner that while the circumferential groove is being formed by using rolling rollers, possible axial bending of the rod is corrected with straightening rollers having a flat peripheral surface, which are disposed at both sides of the rolling rollers in the axial direction thereof. Thus, bending of the rod is suppressed by the straightening effect of the straightening rollers. Accordingly, it becomes unnecessary to perform a straightening operation which would otherwise need to be carried out subsequently. As a result, the processing time can be reduced to a considerable extent, and cost reduction can be attained.

[0046] In addition, a circular cylinder-shaped base metal is subjected to a hardening treatment, and after a circumferential groove has been formed on the base metal by rolling, the base metal is subjected to a surface hardening treatment. With this method, a surface hardening layer can surely be left on the bottom of the groove as well, and it is possible to produce a rod ensured sufficient strength.

[0047] The entire disclosure of Japanese Patent Application No. 2002-54585 filed on Feb. 28, 2002 including specification, claims, drawings and summary is incorporated herein by reference in its entirety. 

What is claimed is:
 1. A cylinder apparatus comprising: a cylinder having a fluid sealed therein; a rod inserted in said cylinder, said rod being formed from a circular cylinder-shaped base metal having a circumferential groove formed by rolling after the base metal has been subjected to a hardening treatment; and an engagement member secured to said rod by engaging in the circumferential groove on said rod.
 2. A cylinder apparatus according to claim 1, wherein a surface hardness of said rod when subjected to rolling is not less than 400 in terms of Vickers hardness (Hv).
 3. A cylinder apparatus according to claim 1, wherein said rolling is carried out in such a manner that while said circumferential groove is being formed by using rolling rollers, possible axial bending of said rod is corrected with straightening rollers having a flat peripheral surface, which are disposed at both sides of said rolling rollers in an axial direction thereof.
 4. A method of manufacturing a rod for use in a cylinder apparatus wherein said rod is inserted into a cylinder having a fluid sealed therein, said cylinder apparatus having an engagement member that is engaged in a circumferential groove formed on said rod, said method comprising the steps of: performing a hardening treatment on a circular cylinder-shaped base metal to harden said base metal; forming said circumferential groove on said base metal by rolling; and performing a surface hardening treatment on said base metal.
 5. A method of manufacturing a rod for use in a cylinder apparatus according to claim 4, wherein a surface hardness of said rod obtained by said hardening treatment is not less than 400 in terms of Vickers hardness (Hv).
 6. A method of manufacturing a rod for use in a cylinder apparatus according to claim 4, wherein said rolling is carried out in such a manner that while said circumferential groove is being formed by using rolling rollers, possible axial bending of said rod is corrected with straightening rollers having a flat peripheral surface, which are disposed at both sides of said rolling rollers in an axial direction thereof.
 7. A cylinder apparatus according to claim 2, wherein said rolling is carried out in such a manner that while said circumferential groove is being formed by using rolling rollers, possible axial bending of said rod is corrected with straightening rollers having a flat peripheral surface, which are disposed at both sides of said rolling rollers in an axial direction thereof.
 8. A method of manufacturing a rod for use in a cylinder apparatus according to claim 5, wherein said rolling is carried out in such a manner that while said circumferential groove is being formed by using rolling rollers, possible axial bending of said rod is corrected with straightening rollers having a flat peripheral surface, which are disposed at both sides of said rolling rollers in an axial direction thereof. 